The prosthetic limb provided with a knee joint normally includes drag generating means such as a pneumatic cylinder, a hydraulic cylinder or a spring cylinder, in order to make effective movements through the knee joint. The pneumatic cylinder has such features that the initial resistance produced at the beginning of bend of knee joint is small and therefore, the burden received by the wearer's hip joint muscle is small. However, since pneumatic pressure is compressed, the force produced by the pneumatic cylinder is smaller than that produced by the hydraulic cylinder or the spring cylinder. Therefore, in order to obtain a force large enough for the prosthetic limb, the size of the pneumatic cylinder must be larger than a certain size. The size-increase of the pneumatic cylinder is inconvenient when a small-sized prosthetic limb is to be made.
On the other hand, the hydraulic cylinder uses oil that is an incompressible fluid and therefore, it can produce a force larger than that of the pneumatic cylinder. The spring cylinder can also produce a force larger than that of the pneumatic cylinder in compliance with the spring constant. Thus, the hydraulic cylinder and the spring cylinder are more advantageous than the pneumatic cylinder from the standpoint of making a small-sized prosthetic limb. On the other hand, the hydraulic cylinder and the spring cylinder have such drawbacks that the prosthetic limb wearer must withstand an increased burden received by his/her hip joint. Because of the reasons just mentioned, use of the prosthetic limb equipped with the hydraulic cylinder is limited to those persons who have strong muscle and body. By reducing the spring force, the prosthetic limb equipped with the spring cylinder can be used by those persons who have comparatively weak muscle and body. However, the prosthetic limb equipped with the spring cylinder having a reduced spring force is not suitable for those who have a strong muscle and body.
For example, Patent Document 1 shows a prosthetic limb equipped with a pneumatic cylinder, Patent Document 2 shows a prosthetic limb equipped with a hydraulic cylinder and Patent Document 3 shows a prosthetic limb equipped with a spring cylinder.    Patent Document 1: Japanese Patent Application Laid-Open No. 2001-137268    Patent Document 2: Japanese Patent Application Laid-Open No. 2002-58689    Patent Document 3: Japanese Patent Application Laid-Open No. S55-130657